In the prior art, substrates of organic resins or plastics are surface coated with various coating compositions to form surface protective films for the purpose of imparting high hardness and mar resistance. For instance, compositions comprising hydrolyzates or partial hydrolyzates of hydrolyzable organosilanes and optionally, colloidal silica are known.
For instance, JP-A S51-2736, JP-A S53-130732 and JP-A S63-168470 disclose coating compositions comprising an organoalkoxysilane or hydrolyzate and/or partial hydrolytic condensate thereof, and colloidal silica, wherein the alkoxy group is converted into silanol in the presence of excess water. These coating compositions form coatings which have a high hardness, good weatherability, and an ability to protect the underlying substrates, but lack toughness. In particular, coatings of at least 10 μm thick tend to develop cracks in such durations as heat curing, removal from the heat curing furnace, outdoor service, and abrupt temperature changes. Although a buffered basic catalyst is used as the curing catalyst from the aspect of storage stability, these coating compositions suffer from a stability problem. The hydrolyzate/condensate of alkoxysilane is composed mainly of relatively low molecular weight compounds, silanol contained in these low molecular weight compounds has a very high reactivity, and the content of silanol is very high. Then condensation reaction of silanol gradually occurs even at room temperature, and the low molecular weight compounds build up their molecular weight with the lapse of time. Then coatings of aged compositions have a low hardness. Sometimes the compositions gel and become unusable as the coating agent. To solve these problems, JP-A 2005-314616 proposes to use a certain basic compound as the curing catalyst to formulate a composition which has shelf stability in the liquid form and exhibits crack resistance, hardness and mar resistance in the cured film state.
However, several problems must be solved before coating films can withstand sunlight and weather over a long time. The coating layers lack an ability to cut UV, allowing a phenomenon to develop that a resin substrate, a primer layer for imparting substrate adhesion or an interface therebetween can be degraded or discolored by UV exposure. Several techniques are proposed to prevent such a phenomenon, including addition of UV absorber to the primer layer, and incorporation via chemical bonds of UV absorptive organic substituent groups into the organic resin of which the primer layer is formed. The UV absorptive organic substituent groups and UV absorbers refer to benzophenone, benzotriazole, triazine and similar substituent groups, and organic compounds containing the same. See JP-A H04-106161, JP 3102696, JP-A 2001-47574, and JP 3841141.
The above technique for cutting off UV is by incorporating an organic UV absorber into a primer layer. Since the primer layer in itself has the main purpose of improving the adhesion between the underlying substrate and a silicone layer, an extra amount of UV absorber loaded therein gives rise to problems such as losses of adhesion and transparency. It is demonstrated in a long-term outdoor exposure test and accelerated weathering test that the UV cut by the primer layer alone is insufficient for preventing degradation and discoloration of organic resin substrates.
One approach taken for compensating for such drawbacks was to add organic UV absorbers to silicone layers as well. However, simply adding UV absorbers to coating compositions results in a less durable coating. That is, the coating fails to sustain the desired UV absorbing property due to bleeding and drainage of UV absorber from the surface during long-term weather exposure. Then organic UV absorbers were developed which are silyl-modified so as to be chemically bondable with siloxane compounds, the main component of the coating layer. See JP-B S61-54800, JP-B H03-14862, JP-B H03-62177, and JP-A H07-278525. This measure improves retentivity since the UV absorber is strongly bound to the siloxane matrix. On the other hand, these coating layers become substantially poor in mar resistance that is essentially desired, or develop noticeable microcracks due to a lowering of flexibility. As discussed above, the organic UV absorbers have the essential drawback that the hardness of silicone film becomes lower as the amount of UV absorber added is increased to enhance weather resistance.
In another attempt, metal oxide nanoparticles having UV shielding property are added to coating compositions so that the compositions may maintain hardness and mar resistance. Known examples are titanium oxide nanoparticles of anatase type (JP-A 2004-238418) and titanium oxide nanoparticles of rutile type (JP 2783417, JP-A H11-310755, JP-A 2000-204301). These coating compositions form UV-shielding coatings which maintain visible light transmitting and mar resistant properties. However, titanium oxide nanoparticles have a photocatalytic activity which cannot be fully suppressed even when they are surface coated with silicon compounds. Additionally, the coatings have insufficient weather resistance in that cracks develop in a relatively early stage in a long-term accelerated weathering test.
It is also known to use zinc oxide nanoparticles as the metal oxide nanoparticles having UV shielding property (see JP-A H11-209695, JP 3347097, and JP-A 2002-60687). In general, the zinc oxide nanoparticles have somewhat poorer UV shielding property than the titanium oxide nanoparticles and accordingly lower photocatalytic activity. However, on account of residual photocatalytic activity, a coating loaded with zinc oxide nanoparticles can not avoid a phenomenon that the coating develops cracks or peels in a weathering test.
JP 3509749 and JP-A 2002-87817 disclose an attempt to suppress photocatalytic activity by coating surfaces of zinc oxide nanoparticles with another oxide. A coating loaded with surface-coated zinc oxide nanoparticles has a longer lifetime in a weathering test than bare zinc oxide nanoparticles. However, the coating is still insufficient as outdoor UV shielding members partly because cracks develop in a long-term weathering test.
In general, visible light transparency is one of important properties of coating compositions for forming weather resistant surface protective coatings. If metal oxide nanoparticles are used as the UV shielding agent, visible light transparency is substantially impaired depending on an average particle size and a tendency to agglomerate. JP-A H11-278838 discloses that when zinc oxide nanoparticles are prepared by a specific method, a dispersion thereof has a smaller particle size and is unsusceptible to agglomeration. A coating composition having this zinc oxide nanoparticle dispersion compounded therein would be highly transparent to visible light although this is not described in Examples.
As discussed above, a number of attempts have been made to improve the weather resistance, mar resistance and other properties of coating compositions. However, there is not available a coating composition whose cured film exhibits mar resistance, UV shielding property, and sufficient weather resistance and durability to withstand prolonged outdoor exposure while maintaining visible light transparency.